1. Field of the Invention
The present invention is generally related to a DC/DC converter and a DC power supply module which contains the same, particularly, to a DC/DC converter and a DC power supply module which contains the same, which are especially suitable for consumer electronics and built-in battery-powered apparatuses.
2. Description of the Related Art
DC/DC converters are utilized in a number of different switching power supply circuits. The basic switching power supply topologies are a step-up DC/DC converter and a step-down DC/DC converter, as disclosed in “Transistor Technology Special—Introduction to Practical Power Electronics—”, published by CQ Publishing Company in August 1998.
An exemplary step-down DC/DC converter is illustrated in FIG. 1A, and designated by reference numeral 100. The DC/DC converter 100 includes a field effect transistor (FET) Q1, a diode D1, a diode D2, an inductor L, a capacitor C, an input terminal 11, an output terminal 12 and a grounded terminal 13, which may be also referred to as a common terminal. The FET Q1 receives an input DC voltage Vin on the drain thereof. The diode D1 has a cathode connected to a drain of the FET Q1, and an anode connected to a source of the FET Q1. The diode D2 has a cathode connected to the source of the FET Q1, and an anode connected to the grounded terminal. The inductor L is connected between the source of the FET Q1 and the output terminal. The capacitor C is connected between the output and grounded terminals. The FET Q1 may be replaced with an insulated gate bipolar transistor (IGBT).
When the FET Q1 is turned on, the input voltage Vin is applied to the inductor L, and power is delivered to the output terminal. An output voltage Vout is Vin-VL, where the VL is the voltage across the inductor L. The output voltage Vout also is formed across the capacitor C, thus the capacitor C charges and the output voltage increases each time the FET Q1 is turned on.
When the FET Q1 is turned off, the voltage across the inductor L reverses due to inductive flyback, thus making the diode D2 forward biased. A circuit loop generated by the diode D2 allows the energy stored in the inductor L to be delivered to the output terminal, wherein an output current is smoothed by the capacitor C. In this way, the input voltage Vin is converted to output voltage Vout. The output voltage can be controlled by varying the duty-cycle of the control voltage on the gate of FET.
The inductor L and the capacitor C are used for smoothing the ripple of the output voltage Vout. The FET Q1 is switched at a relatively high frequency to produce a chopped output voltage, however, the inductor L and the capacitor C operate together as an LC filter to produce a relatively smooth output voltage having a DC component with a reduced ripple voltage. The inductor and the capacitor are used to suppress the ripple voltage.
An exemplary step-up DC/DC converter is illustrated in FIG. 1B, and designated by reference numeral 101. The step-up type DC/DC converter 101 includes a field effect transistor (FET) Q2, a diode D1, a diode D2, an inductor L, a capacitor C, an input terminal 11, an output terminal 12 and a grounded terminal 13. The inductor L is connected between the input terminal 11 and a drain of the FET Q2. The FET Q2 receives Vin-VL on the drain terminal thereof, wherein the Vin is the input DC voltage and VL is the voltage across the inductor L. The diode D1 has a cathode connected to a drain of the FET Q2, and an anode connected to a source of the FET Q2. The diode D2 has a cathode connected to the output terminal, and an anode connected to the drain of the FET Q2. The capacitor C is connected between the output terminal and the grounded terminal. The FET Q2 may be replaced with an insulated gate bipolar transistor (IGBT). An operation of the DC/DC converter 101 is similar to that of the DC/DC converter 100.
Bipolar transistors are generally used as switching devices or amplifiers in a number of different circuit applications. Important requirements for bipolar transistors as switching devices are a high withstand voltage, a low voltage at its on-state, a high speed switching, and a high current gain. Conventional bipolar transistors are disclosed in Japanese Unexamined Patent Applications No. P2001-338928A and P2002-299602A.
An NPN bipolar transistor disclosed in No. P2002-299602A is illustrated in FIG. 2, and designated at reference numeral 200. The NPN bipolar transistor 200 includes a collector electrode 28, a collector 23 formed on the collector electrode 28, a base 24 formed on the collector 23, emitters 25 formed on the base 24, base electrodes 26 formed on the base 24, and emitter electrodes 27 formed on the emitters 25. A collector 23 consists of an N+ substrate 22, an N type region 23a and an N− type region 23b. The N+ substrate 22 is located at the bottom of the collector 23, and the region 23a is formed on the N+ substrate 22. The region 23b is formed between the region 23a and the base 24. A dopant concentration of the region 23b is higher than that of region 23a. 
The structure 200 improves the current gain because the region 23b prevents hole injection into the region 23a. 
An NPN silicon germanium transistor disclosed in Japanese Laid Open Patent Application No. P2001-338928A and its corresponding U.S. Pat. No. 6,423,989 is illustrated in FIG. 3, and designated at reference numeral 300. The NPN silicon germanium transistor 300 includes a collector electrode 37, a collector 31,32 formed on the collector electrode 37, a base 33,34 formed on the collector 31,32, an emitter 35,36 formed on the base, base electrodes 38 formed on the base 34 and emitter electrodes 39 formed on the emitter 36. For high current gain and high breakdown voltage, the base is formed of two layers: a P type silicon germanium layer 33 within which the lifetime and the mobility of the electrons are relatively reduced, and a P type silicon layer 34 within which the lifetime and the mobility of electrons are relatively increased. A germanium concentration in the base increases as distance from the collector decreases. This structure effectively improves transfer efficiency of electrons from the emitter 35 to the collector 32. As a result, the current gain is enhanced.
In many applications, the power loss of a DC/DC converter is to be suppressed to be as small as possible. The power loss of the DC/DC converter is generated at the on-state and transient state of turn-on and turn-off. Low saturation voltage at the on-state and short transient time at turn-on and turn-off are required when a switching semiconductor device is used in a DC/DC converter. However, a conventional FET or IGBT is not able to realize all these performance requirements in one device at the same time. Also, the on-state resistances of FETs and IGBTs tend to be high. The high on-state resistance causes power loss while FETs or IGBTs are turned on. Current FETs have only one advantage of high switching speed or low resistance; no FET is known having both of these advantages. IGBTs are useful for large capacity applications; however their transient times at turn-on or turn-off are as large as one microsecond, and the on-state voltages at the saturation state are as large as 1.5 voltage.
The slow switching speed of FETs and IGBTs limits the switching frequency of the DC/DC converters to below 1 MHz. The switching frequency limitation undesirably increases the ripple of the output voltage. In order to suppress the ripple voltage, a capacitor of large capacitance and an inductor of large inductance have to be used, which prevents a volume or size reduction of the DC/DC converters. The large inductance needs a coil of many turns, and the resistance and resistance loss of the inductor undesirably increases.
Accordingly, there is a need for a DC/DC converter which has a suppressed power loss and a reduced volume using a new switching semiconductor device.